Coating substrates with metals using the sputtering or disintegration process is relatively simple since metals are good electrical conductors. It is significantly more difficult to coat substrates with oxide layers which have no or only very low electrical conductivity. In order to be able to deposit also oxides and other dielectrics on a substrate despite this difficulty, metal particles are generated through means of dc current sputtering which subsequently are converted in a reactive atmosphere to oxides and deposited on the substrate.
Converting the metal particles into oxides in this process takes place in the immediate vicinity of the substrate and at a distance from the sputtering cathode in order to also avoid the deposition of oxides on this cathode with a concomitant decrease of the sputtering rate. Nevertheless, in practice it is not possible to keep the cathode entirely free of oxides so that the sputtering rate gradually decreases significantly.
If sputtering is carried out with magnetron cathodes, the problem arises that there where the magnetic field lines have their maxima of curvature, sputtering is most intense so that a sputter groove originates. The intense sputtering at these sites prevents the deposition of oxides. On those regions of the target which are not sputtered off at all or only very slowly, however, nonconducting dielectric films form under the influence of reactive gases. These growing regions are charged electrostatically, are starting points of spontaneous flashovers on the target surface and, as a consequence, of flashovers between target and plasma or target and the substance surrounding the target. During these flashovers the cathode current must briefly be reduced so that the discharge arc can be quenched which, however, entrains further instabilities until the discharge finally ceases.
In dc current magnetron sputtering, in contrast to pure diode sputtering, partial covering of the target with reaction products cannot be prevented, at best it can only be kept low by optimizing the magnetic field.
A starting point for the solution of the problem is obtained, if, instead of a dc voltage a high frequency (HF) ac voltage between target electrode and substrate is applied. In this HF magnetron sputtering in a reactive atmosphere no flashovers occur on the target surface and no electrostatic charging takes place. However, in pure HF sputtering the sputtering rate is relatively low.
It is, however, also known to deposit tantalum and tantalum oxide on a substrate using a dc current sputtering process in which an HF voltage is superimposed on the dc voltage (F. Vratny: Deposition of tantalum and tantalum oxide by superimposed RF and DC sputtering; Conference Paper on Thin Film Dielectrics, Oct. 7 to 11, 1968, Reprint from J. Electrochem. Soc., 114-5, 505, 1967). The combined dc current/ac current field increases the plasma density and prevents the formation of a dielectric film on the cathode during the reactive sputtering. Thereby it becomes possible to achieve at sputtering pressures of 0.5 to 2 millitorr a tantalum deposition rate of approximately 80 .ANG./min and a twofold increase of tantalum deposition rate at pressures in the range of 10 to 20 millitorr. A deposition rate of 50 to 100 .ANG./min obtains for coatings of Ta.sub.2 O.sub.5 and MnO.sub.x during reactive sputtering in pure oxygen.
The increase of the deposition rate can be explained by the fact that in an applied HF field the charged particles perform an oscillating motion. The electrons which migrate under the influence of the superimposed field cover a greater distance than the electrons which are only exposed to a dc field. The greater distance increases the probability of electron gas/atom collisions which leads to an increase of the current density of the positive ions at the cathode at a given pressure. This, in turn, effects an increase of the sputtering rate and layer deposition. How the electrons react in the gas depends on the gas pressure or the free path length of the electron, the frequency of the HF field and the apparatus of the electrodes. At low pressures if the mean free path length is greater than the distance between the electrodes, the electrons are excited and traverse the space between the electrodes nearly entirely without collisions with the gas. For example, the electrons in argon with an energy of 0.4 eV at a pressure of 10 millitorr have a mean free path length of 10 cm which corresponds approximately to the distance of conventional electrodes. At pressures at which the mean free path length of an electron is less than the distance of the electrodes and the frequency of the field is less than the collision frequency of the gas, the electrons collide several times with each oscillation and have a tendency to move with the phase of the field. Examples for this are the low-frequency ac current sputtering as well as the low-frequency dc current/ac current sputtering in which the electrons bombard the cathode and the substrate in succession. At high frequencies the electrons are able to carry out numerous oscillations of small amplitude between the gas collisions. In this case the electron cloud appears stationary, which results in an intense plasma which can be drawn off with a superimposed dc field. At still higher frequencies, for example in the microwave range, the electrons are under the influence of a standing wave with electric and magnetic components. Due to this influence the electrons are distributed in space in accordance with the spatial conditions, i.e. as a function of the electrode dimensions and the frequency which generates the standing wave.
The presence of the HF field functions, in addition, to prevent a deposition of a dielectric coating on the cathode during reactive sputtering with an electro-negative gas. Since the ion density can be maintained through the HF field, the bombardment at the cathode decreases the probability of the formation of a significant insulator coating. If an insulator coating should be produced, the HF-induced charge at the surface would maintain the sputtering process and, in addition, decrease the process of insulation formation. Due to the greater ionization probability and the decrease of the breakdown strength of the gas, the HF field permits operation at lower sputtering pressures than normally occur during diode sputtering.
The above described known apparatus refers to diode sputtering or diode disintegration. Diode sputtering, however, has the disadvantage that for numerous applications it has too low a deposition rate, and specifically even if ac current superposition is made use of. The above mentioned sputtering with a magnetron cathode yields substantially higher sputtering rates.
It is also known to combine magnetron sputtering with microwave radiation (U.S. Pat. No. 4 610 770=EP 0 148 504). The microwave radiation in this case is intended to prevent the target being sputtered off only where the toroidal field lines of the magnetron electrode are located. The sputtering area, thus, is increased considerably in order to increase the sputtering rate and to keep the target from being eroded not only in a narrowly limited region. In order to achieve this goal a mirror magnetic field is generated with only two permanent magnets which are arranged on a common axis and coaxially to each other. In this manner a magnetic flux distribution is obtained in which the lines of the magnetic flux expand in the intermediate region between the magnets and contract again in the vicinity of the magnets. Of disadvantage here is the costly microwave irradiation with hollow waveguides or similar means as well as the complicated structuring of the permanent magnets.